1. Field of the Invention
The present invention relates to a primary radiator attached to a satellite broadcast reflective antenna or the like.
2. Description of the Prior Art
FIG. 4 is a drawing showing the configuration of a conventional primary radiator, which comprises a circular waveguide 10 having a conical horn part 10a at one end thereof and a watertight cap 11 attached to an open end of the horn part 10a. The watertight cap 11, made of a dielectric material, comprises a dielectric part 11a covering the open end of the horn part 10a and a cylindrical projection part 11b projecting inside from a central part of the dielectric part 11a. The thickness t of the dielectric part 11a is set so as to provide a sufficient strength but is set thinner than the wavelength of radio waves propagating through it. The diameter d and height h of the projection part 11b are set to proper dimensions, and letting the wavelength of radio waves propagating through the projection part 11b be xcexc, the height h is set between about xe2x85x9cxcex and xc2xdxcex.
The primary radiator configured in this way is placed in the vicinity of the focus position of a reflecting mirror of a satellite broadcast reflective antenna, and radio waves from a satellite, reflected on the reflecting mirror, travel from the horn part 10a to the waveguide 10 via the watertight cap 11. At this time, since the radio waves reflected on the surface and back of the dielectric part 11a are canceled by the projection part 11b, radio wave reflection in the watertight cap 11 is reduced so that a satisfactory reflection loss property is obtained. The open end of the horn part 10a covered with the watertight cap 11 having a sufficient strength prevents rainwater, dust, and the like from invading the horn part 10a. 
Since a watertight cap is exposed to rain water and sunlight, it is desirable to make it of a dielectric material having excellent weatherability such as AES resin, ABS resin, and the like. However, this has been a problem in that, since this type of material generally has a high dielectric loss, in the case where the dielectric part 11a is formed integrally with the projection part 11b to constitute the watertight cap 11 as in the conventional example described above, although a satisfactory reflection loss property can be obtained, the watertight cap 11 having a high dielectric loss increases loss.
The present invention has been made in view of the above circumstances of the prior art and provides a primary radiator capable of achieving both low reflection and low loss.
To achieve the above object, a primary radiator of the present invention comprises: a waveguide having a horn part at one end thereof; a watertight cap attached to an open end of the horn part; and a reflection preventing member disposed within the horn part at a fixed interval from the watertight cap, wherein the reflection preventing member is made of a dielectric material having a lower dielectric loss than the watertight cap.
With this construction, since reflection in the watertight cap and reflection in the reflection preventing member cancel each other out, a satisfactory reflection loss property can be obtained, and since the reflection preventing member is made of a dielectric material having a lower dielectric loss than the watertight cap, dielectric loss due to the watertight cap is suppressed and low loss can be achieved.
In the above configuration, whether radio waves from a satellite are linearly polarized waves or circularly polarized waves, particularly in the case of a primary radiator that converts circularly polarized waves to linearly polarized waves, it is desirable to dispose a dielectric plate used as a 90-degree phase element within the waveguide and provide the reflection preventing member integrally with the dielectric plate, whereby a circularly polarized wave primary radiator with low reflection and low loss can be realized.
In the above configuration, an impedance conversion part having a stepwise gap whose depth is about one-fourth an in-tube wavelength is formed at each end of the dielectric plate and a reflection preventing member formed continuously to one impedance conversion part is formed projectingly to the inside of the horn part, whereby the overall length of the dielectric plate can be reduced and the primary radiator can be miniaturized.